Sporocladopsis jackii is a new species of green seaweed from eastern Canada and Maine. The species is abundant in late summer and is a conspicuous part of the epizoic flora on living shells of the mud snail, Ilyanassa obsoleta. The alga comprises a densely packed, branched filamentous base from which arise numerous unbranched to sparsely branched erect free axes. Multicellular attenuate hairs are rare to common and terminal on erect axes. Sporangia are clavate, terminal or lateral, usually borne singly, and produced abundantly in summer. Prior floristic accounts from eastern North America may have included S. jackii under the name Pilinia rimosa, now known to refer to a phaeophyte. Pulse amplitude modulation fluorometry of chlorophyll fluorescence (Phyto-PAM) for pigment analysis confirm microscopic observations of the communities living on snails as consisting of primarily Cyanobacteria and Chlorophyta.